Blueberry plants are a rewarding addition to many gardens, offering delicious fruit and attractive foliage. Their successful growth and abundant fruit production are closely tied to the temperatures they experience throughout the year. Understanding how these plants respond to varying degrees of cold and heat is important for gardeners aiming for a successful harvest.
Blueberry Temperature Fundamentals
Blueberries thrive within a specific temperature range for active growth, generally performing well between 60°F and 70°F (15°C to 21°C) during their growing season. Some studies indicate that optimal growth temperatures for certain cultivars can be as high as 32.6°C (90.7°F) for ‘Bluecrop’ or 31.8°C (89.2°F) for ‘Brigitta’. A period of cold temperatures, known as “chilling hours,” is necessary for these plants to properly develop and produce fruit.
Chilling hours refer to the cumulative time a blueberry plant is exposed to temperatures typically between 32°F and 45°F (0°C to 7°C) during its winter dormancy. This cold period is a signal for the plant to break dormancy, ensuring proper bud development and flowering in the spring. Without sufficient chilling, flower buds might not open or may open unevenly, leading to reduced fruit yield. Chilling hour requirements vary among varieties, typically ranging from 300 to 1000 hours, though some need as few as 200 or as many as 1400 hours.
Cold Tolerance and Winter Protection
Blueberry plants exhibit varying degrees of natural cold hardiness, depending on the variety. During deep winter dormancy, highbush blueberry flower buds can withstand temperatures as low as -20°F to -30°F, while rabbiteye varieties, which are generally less hardy, may be damaged below 0°F. As flower buds begin to swell and develop in late winter or early spring, their tolerance to cold temperatures decreases. For instance, once individual flowers start to emerge, temperatures below 20°F can cause damage.
Late spring frosts, occurring after dormancy has broken and new growth has emerged, pose a threat to blueberry blossoms and young fruit. Temperatures dropping below 28°F can lead to economic losses for highbush blueberries. Protection methods are used to mitigate this risk. Mulching around the base of the plants with organic materials like pine needles, wood chips, or shredded leaves helps insulate the roots and regulate soil temperature, preventing damaging freeze-thaw cycles. A 4 to 8-inch layer of mulch is recommended, ensuring it’s not packed too tightly around the plant’s crown.
Physical covers, such as floating row covers or nursery foam, can provide several degrees of temperature protection during frost events. These should be applied in the late afternoon to trap warmth and secured to prevent wind from dislodging them. For container-grown blueberries, moving them to a sheltered area, bundling them together, and mulching heavily can offer protection. Overhead irrigation is an effective method, where water is continuously applied during freezing temperatures. As water freezes, it releases latent heat, maintaining the plant’s temperature above damaging levels, and this method can protect plants down to 20-23°F.
Heat Tolerance and Summer Protection
High temperatures can cause stress to blueberry plants, potentially impacting their growth and the quality of their fruit. While blueberries can tolerate temperatures up to 104°F (40°C) during summer, prolonged exposure to high heat, especially during flowering and fruit development, can affect fruit quality and may even damage pollen. Some research indicates that temperatures above 35°C (95°F) can lead to a decrease in photosynthesis and transpiration rates.
Maintaining consistent soil moisture is important during hot periods, as blueberries are shallow-rooted and susceptible to drought stress. Regular irrigation, especially through drip systems or microsprinklers, helps ensure the plants have adequate water. During heat waves, overhead irrigation can be used to actively cool the plants and the surrounding air. Running sprinklers for 15 minutes every hour when temperatures exceed 85°F (29°C) can reduce air temperature by 5-10°F, helping to prevent heat damage to blooms and fruit.
Providing partial shade in regions with intense summer sun can help mitigate heat stress. This can be achieved through the use of shade cloths or by planting blueberries in locations that receive some afternoon shade. Proper soil health, characterized by good drainage and high organic matter content, further supports the plant’s ability to withstand heat by improving water retention and aeration.
Selecting Varieties for Your Climate
Choosing the appropriate blueberry variety is important for successful cultivation, as different types have specific temperature requirements.
Highbush Blueberries
Highbush blueberries, including Northern Highbush and Southern Highbush, are widely grown. Northern Highbush varieties are known for their cold hardiness and require 800 to 1000 or more chilling hours, making them suitable for colder climates. Examples include ‘Bluecrop’ and ‘Patriot’, which are hardy to low temperatures.
Southern Highbush blueberries are hybrids developed for warmer climates, possessing lower chilling hour requirements (as few as 150-800 hours) and greater heat tolerance. Varieties like ‘Emerald’, ‘Jewel’, ‘Misty’, and ‘Sunshine Blue’ are well-suited for regions with milder winters and hotter summers. These varieties can tolerate partial shade, which is beneficial in hot environments.
Other Blueberry Types
Rabbiteye blueberries are a type resilient in warm and humid conditions, requiring several hundred chilling hours. Cultivars such as ‘Climax’, ‘Brightwell’, and ‘Premier’ are known for their adaptability to warmer regions.
Lowbush blueberries are smaller and more compact, thriving in very cold climates, with some varieties like ‘Northcountry’ and ‘Northsky’ being cold-hardy to zone 3. Half-High blueberries are a cross between Highbush and Lowbush, offering cold tolerance while producing larger fruit than pure Lowbush varieties.
Matching chilling hour requirements and heat tolerance of a chosen variety to the local climate is important for a productive blueberry patch.